1. Field of the Invention
The present invention relates to an amplifier, and especially to a high-frequency amplifier.
2. Description of the Related Art
In recent broadband communication systems, a signal is used whose peak power is larger than average power by about 10 dB. In such a case, to transmit information without an error, a high-frequency amplifier which can transmit peak power that is 10 dB or more higher than the average transmission power during the transmitting stage has to be used.
Generally, an amplifier has lower power efficiency, as the ratio of the peak power to the average transmission power (called “back-off”) becomes larger. On the other hand, because of an increase in the level of environmental awareness, lower power consumption is also required in radio communication systems, and especially, it is desired that the high-frequency amplifier increases efficiency of power consumption rate.
Approaches to increasing the efficiency of the high-frequency amplifier include a method, such as an envelope tracking system or an envelope elimination and restoration (EER) system that makes a drain voltage of an FET (Field effect transistor), which is an amplifying element of an amplifier, change in synchronization with the envelope of a signal. For example, a method, as disclosed in Japanese Patent Application Laid-Open No. 2006-093874, makes the back-off small to increase efficiency, by lowering the drain voltage to decrease the peak power of an amplifier, when a signal level is low.
Here, a high-frequency amplifier accomplishes impedance matching by providing a matching circuit in the input/output portions of a semiconductor device for amplification such as an FET or a bipolar transistor. However, the impedance of the semiconductor device generally changes, when bias conditions are changed, and therefore, even if the matching circuit is optimized under some bias conditions, it is not necessarily optimized under different bias conditions.
Then, as a technique to provide matching in conformity with the bias conditions, National Publication of International Patent Application No. 2003-524988 discloses an envelope tracking amplifier corrects the impedance of input/output matching circuits that corresponds to an envelope of an input signal.
The amplifier described in National Publication of International Patent Application No. 2003-524988 implements impedance correction of the input/output matching circuits correspondingly to the envelope of the input signal.
In the above amplifier, but voltage control of a semiconductor device for amplification but voltage control is applied to a semiconductor device used as an amplifying device to vary the bias voltage and control voltage in order to variably control the impedance of the input/output matching circuits that are configured independently of each other, so that optimized matching circuits are not provided when timing of each voltage control is shifted from each other, which presents a problem that good efficiency control cannot be carried out.